The mode by which HTLV-1 is spread among people, its persistence in individuals, and the slow course and outcome of infection, are the outward manifestation of complex molecular interactions between the virus and host cells. We believe that by characterizing the virus infectious cycle and its dependence on cellular processes and pathways, we will be better able to identify targets for therapeutic interventions. We are using genetic methods combined with cell and molecular biological approaches to examine the assembly and transmission of HTLV-1 particles between cells, virus replication and gene expression, and virus interactions with intrinsic cellular restriction factors. Our understanding of the HTLV-1 infectious cycle lags behind that of HIV-1, because HTLV-1 does not spread in established T-cell lines and is more difficult to study in cell culture. More than a decade ago, we constructed one of the first infectious molecular clones of HTLV-1, and later we developed vectors and cell culture methods that made it possible to analyze and quantify HTLV-1 infection and replication in vitro. These tools are now an essential complement to biochemical and molecular methods that are used to study HTLV-1 infection and replication. With respect to HTLV-1 assembly and release, we are examining virion targeting to specific membrane microdomains via interactions between Gag and cellular proteins and lipids; virus particle assembly mediated by Gag interactions with NEDD4, a cellular ubiquitin ligase; and defining the pathways that coordinate Gag and Env trafficking to sites of virus assembly. We are actively studying mechanisms of retroviral inhibition by, and resistance to, cellular restriction factors belonging to the APOBEC family of cytidine deaminases. And finally, we are constructing molecular clones and vectors for the other human deltaretroviruses, HTLV-2, -3, and -4, to take advantage of the biological and genetic diversity among these viruses for comparative analyses. [Corresponds to Derse Project 1 in the April 2007 site visit report of the HIV Drug Resistance Program]